© 2005 Nature Publishing Group T he announcement by Kasahara and Kato of a new redox-cofactor vitamin for mammals 1 , pyrroloquinoline quinone (PQQ), was based on their claim that an enzyme, predicted to be involved in mouse lysine metabolism, is a PQQ-dependent dehydrogenase. However, this claim was dependent on a sequence analysis using data- bases that inappropriately label ȋ-propeller sequences as PQQ-binding motifs. What the evidence actually suggests is that the enzyme is an interesting novel protein that has a seven-bladed ȋ-propeller structure, but there is nothing to indicate that it is a PQQ-dependent dehydrogenase. In bacteria, PQQ is an essential cofactor for various dehydrogenase enzymes known as quinoproteins 2 . Nutritional experiments have indicated some (unknown) metabolic or nutritional role for PQQ in mammals 1,3,4 , but it cannot be accepted as a vitamin until it is proved to be required by an enzyme as an essential cofactor; this is the key evidence that Kasahara and Kato 1 claim to have provided. In the course of a study on bipolar disorder (see www.brain.riken.go.jp/labs/mdmd/pqq/ index-e.html), these authors cloned a mouse gene encoding a protein (U26) with some similarity to yeast aminoadipate reductase (LYS2) 5 ; they proposed that mouse U26 could be involved in one of the important first steps in the degradation of dietary lysine, acting as a PQQ-dependent adipic 6- semialdehyde dehydrogenase. As would be expected from the method used by Kasahara and Kato in searching for the LYS2 analogue 1 , the U26 sequence con- tained no carboxy-terminal NAD(P)-bind- ing domain. They noted from sequence analysis that U26 has an alternative carboxy- terminal domain that contains seven repeats of the ‘PQQ-binding motif ’ that is conserved among bacterial PQQ-dependent dehydro- genase enzymes, leading to the conclusion that mouse U26 could be a PQQ-dependent dehydrogenase. However, this conclusion is based on the misconception that the Smart and Pfam brief communications arising NATURE | VOL 433 | 27 JANUARY 2005 | www.nature.com/nature E10 databases are able to recognize PQQ-bind- ing sites even when, as in this case, there is negligible sequence similarity to known PQQ-dependent enzymes. The ‘sites’ wrongly identified by the databases do not represent PQQ-binding sites but represent the ȋ- sheets that form the ‘blades’ of the ‘propeller fold’ that happens to be a feature of all PQQ- dependent dehydrogenases, whose main structure is a superbarrel made up of either six or eight ‘propeller blades’ (Fig. 1). The propeller fold is not related in any direct way to PQQ binding 2,6 , and these folds are found in many other types of protein, which have extreme functional and phylo- genetic diversity 7 . We contend that there is still no com- pelling evidence for a PQQ-dependent enzyme in the mouse and that the authors’ announcement of a new vitamin was there- fore premature. Leigh M. Felton, Chris Anthony School of Biological Sciences, University of Southampton, Southampton SO16 7PX, UK e-mail: c.anthony@soton.ac.uk doi:10.1038/nature03322 1. Kasahara, T. & Kato, T. Nature 422, 832 (2003). 2. Goodwin, P. M. & Anthony, C. Adv. Microb. Physiol. 40, 1–80 (1998). 3. McIntire, W. S. Annu. Rev. Nutr. 18, 145–177 (1998). 4. Steinberg, F. et al. Exp. Biol. Med. 228, 160–166 (2003). 5. Ehmann, D. E., Gehring, A. M. & Walsh, C. T. Biochemistry 38, 6171–6177 (1999). 6. Anthony, C. & Ghosh, M. Progr. Biophys. Mol. Biol. 69, 1–21 (1998). 7. Paoli, M. Progr. Biophys. Mol. Biol. 76, 103–130 (2001). Reply: T. Kasahara and T. Kato reply to this communication (doi:10.1038/nature03324). Biochemistry Is pyrroloquinoline quinone a vitamin? T he announcement by Kasahara and Kato 1 of pyrroloquinoline quinone (PQQ) as a ‘new’ vitamin has received considerable attention. We have since attempted to reproduce the findings on which their conclusion is based, namely that defects in lysine metabolism occur in PQQ-deprived rodents. However, we find that the activity of Ȋ-aminoadipic acid-Ȏ- semialdehyde (AAS) dehydrogenase in liver and plasma levels of Ȋ-aminoadipic acid (AAA), both of which act as indicators of lysine degradation in mammals, are not affected by changes in PQQ dietary status. Our results call into question the identifica- tion of PQQ as a new vitamin. The observations of Kasahara and Kato 1 were of particular interest to us because their experimental model was based on our find- ings that mice that are fed chemically defined diets and deprived of PQQ often show signs of reproductive failure and compromised neonatal growth 2,3 . Using previously described assay conditions 4 , we found that Biochemistry Role of PQQ as a mammalian enzyme cofactor? Arising from: T. Kasahara & T. Kato Nature 422, 832 (2003) Figure 1 The superbarrel structure of the Ȋ-subunit of the PQQ-dependent methanol dehydrogenase 6 . This is simplified to show the ȋ- sheets (labelled W1–W8) that form the ‘propeller blades’ of the propeller fold; each blade is made up of four antiparallel strands (labelled A–D). It can be seen that these eight ȋ-sheets have no specific role in binding the PQQ molecule (shown as ball-and-stick model). The calcium ion is shown as a small sphere.